1. Field of the Invention
This invention relates generally to flat panel color displays and, more particularly, to displays in which the image is the result of a mosaic of pixel regions.
2. Description of the Related Art
Liquid crystal mosaic display technology is being developed as a possible successor to color cathode ray tubes (CRTs) in many display applications, including those applications in the avionics field. This technology offers important advantages such as higher reliability along with reduced power, size and weight. But in the current state of development of the liquid crystal technology, capability of this technology for the rendering of an image falls short of the image capability achievable using CRT technology. This invention addresses three specific problem areas still remaining in liquid crystal mosaic displays: color definition; image resolution; and display brightness. In terms of color definition, the liquid crystal mosaic display color rendition suffers from effects similar to those observed on a misaligned CRT display tube. The primary hues, the red, green and blue colors, do not blend properly. A white line, for example, appears to have multicolored fringes, symptomatic of deficient color synthesis. Part of the problem can be attributed to the symbol generator which controls the formation of graphics on the flat panel. However, part of the problem can also be attributed to the display itself, a contribution addressed by this invention.
In terms of image resolution, graphic symbols and lines appear excessively jagged or discontinuous on color mosaic displays, especially when compared with lines drawn on calligraphic color CRT systems. Again part of the image resolution problem can be attributed to the symbol generator while the display panel itself also provides a contribution. A major part of the contribution from the display panel is the result of the presence of blue pixels as part of the display pixel mosaic. Referring now to FIG. 1A, the low degree of spatial sensitivity that the human visual system has for blue light as compared to the other primary colors is illustrated. The eye's peak response to blue light occurs at about one half the frequency of peak response for the red radiation and half again the frequency for green radiation. This result indicates that blue radiation contributes only a minor amount to image shape and spatial detail. As a result, blue pixels on the display surface of the panel tend to degrade the overall resolution capability of color mosaic displays, a feature addressed by the present invention.
With respect to display brightness, the origin of the problem can be attributed to both the pixel arrangement of the panel and the current backlight technology used in liquid crystal displays. The backlight technology includes the lamp and the electronics controlling the backlight lamp. The chief figure of merit for achieving a given level of brightness is how much power is needed to achieve that brightness level. Research is being aggressively pursued to make backlight technology more efficient.
The present invention, however, addresses the brightness problem from a different perspective. Once again, the pixel arrangement on the surface of the flat panel display can account for a considerable portion of the problem. Blue pixels contribute little to the total perceived luminance of the panel display. The photopic response of the eye accounts for this phenomenon. FIG. 1B illustrates that red and green radiation provide a larger contribution to perceived brightness than blue radiation. Blue radiation can typically provide only about a ten percent contribution to the overall brightness of the panel.
Referring next to FIG. 2, the effect of having blue pixels occupying space in the pixel arrangement is shown. Wherever a blue pixel is present, the effect on the pattern of pixels is to occlude the perceivable luminance passing through the display surface. No appreciable contribution to luminance capability is available at the sites of the blue pixels. As a result, these blue pixel regions of FIG. 2 can be considered as black regions. These regions occupy thirty percent of useful area in a typical Red/Green/Blue (RGB) pixel mosaic arrangement.
In order to compete successfully with the cathode ray tube technology in a multiplicity of applications, the liquid crystal mosaic displays must evolve to the point where they efficiently achieve enough brightness to prevent bright sunshine from washing out displayed information. Additionally, they must also exhibit higher resolution and improved color mixture attributes for higher quality imagery to be displayed. Achieving these goals has proven difficult in the past.
A wide range of techniques have been implemented in flat panel display technology to alleviate the problems described above. Listed below is a description of the principal approaches for solving color definition, image resolution and display brightness problems in the liquid crystal mosaic displays.
Generally, color image synthesis in liquid crystal mosaic displays use either additive or subtractive techniques (cf. the above identified related application). Additive techniques use spatial proximity, temporal superposition or spatial superposition techniques to mix primary hues into different colors. Additive spatial proximity methods are the most common approach used in liquid crystal flat panel technology. FIG. 3 illustrates the basic technique of spatial proximity. Small dots (pixels) of primary colors, typically red, green and blue, are evenly dispersed across the surface of the flat panel display. If the dots (pixels) are small enough and close enough, then the eye fuses or integrates the contribution of each color dot together with its neighbors. The additive method can achieve enhanced resolution by making the pixels smaller and more densely packed. Additionally, the differently colored pixels can be arranged into different patterns, in hopes of striking a better fit with the characteristics of the human visual system. Full color imagery is therefore perceived. Excellent resolution can result because each pixel is capable of full color control and full luminance control. Additive spatial proximity, the method generally preferred throughout the industry, suffers three serious drawbacks, outlined above in the problem discussion. Color definition is faulty in the case of computer generated imagery (unless signal processing methods are used) resulting in color fringing and rainbows effects. As the pixels are made smaller, color integration is improved but light output is worsened because a greater percentage of the primary display area gets consumed by address lines and interconnecting conductors. In addition, blue contributes very little to perceived brightness yet consumes typically one quarter to one third the active display area as indicated previously. Blue also detracts from resolution capability, limiting edge definition and image sharpness. The three principal problems with this approach then are: (1 ) poor color integration and (2) wasted luminance and (3) wasted resolution.
In additive temporal superposition methods, the primary hues are rapidly sequenced before the eye. FIG. 4 shows one possible sequence. First, the red portion of the image is flashed on the flat panel display, then the green portion of the image is flashed on the flat panel display and, a short time later, the blue portion of the image is flashed on the flat panel display. Successful color synthesis using this temporal additive technique depends on the limited temporal frequency response of human vision. If the sequencing occurs rapidly enough, the eye cannot discern the separate primary hues, but, instead, perceives their overall integrated image. Temporal superposition suffers from smearing effects, jitter and image instability as the observer shifts his viewing position rapidly or vibration induces similar motion. In addition, todays liquid crystal materials exhibit such slow optical response times, rapid temporal sequencing using them is virtually impossible.
In additive spatial superposition methods, separate images, each comprised of only one primary hue, are optically fused into one full color image. Typically three images, corresponding to red, green and blue hues, are used. These separate images are formed from three separate image sources. The output images of these three sources are then fused by optics into one full color image to be viewed by the observer (cf. FIG. 5). Excellent resolution is typical of this approach because each pixel is capable of full color and full luminance control. Brightness can also be high since three image forming sources are operated in parallel. Additive spatial superposition techniques suffer from complexity problems and performance difficulties. These systems also tend to be prohibitively large for many applications, especially those of the aerospace market. Cost generally rises due to the fact that three separate imaging devices are needed. Then additional hardware must be used to combine the three images. Frequently, this hardware must be extremely precise and rigid to maintain color purity.
In subtractive display apparatus (illustrated in FIG. 6), white (broad band) radiation is passed through successive layers of complimentary color filters, each layer being electrically controlled for absorbing a well-defined region of the spectrum. By modulating the voltage applied to each layer, different portions of the white light spectrum can be occluded or, in the alternative, be allowed to pass through unimpeded. This spectral control, the ability to withdraw selectively different portions of the spectrum, can be used to synthesize full color imagery. Resolution can be excellent with this approach because full color control is available at the site of each pixel. Subtractive methods suffer from an expected higher cost, parallax effects and complex methods for color control. At least three and possibly four separate liquid crystal panels are needed to make a subtractive superpositional liquid crystal display. Currently, this additional complexity is viewed as too costly. In addition, parallax can be troublesome using this technique. As the viewing angle is changed with respect to the display, each layer of pixels in the panel is viewed from a slightly different position. Pixels on different layers of the display will be observed to move with respect to each other. Lines can vary substantially in perceived thickness, due to head motion alone. Effects such as these, which are functions of viewing angle, are unacceptable for many (e.g., aerospace) applications. Finally, color control has proven to be particularly troublesome to date. Experiments indicate that, unless better dyes, backlighting or even a fourth layer can be developed, colors cannot be tracked over a broad range of ambient lighting conditions. The difficulty lies in the complex interrelationships between hue and luminance. One interferes with the other in a non-linear manner which currently has proven very difficult to predict.
A need has therefore been felt for a liquid crystal display unit that provides increased display brightness, increased image resolution and better color reproduction.